Subterranean heaters have been used to heat subterranean geological formations in oil production, remediation of contaminated soils, accelerating digestion of landfills, thawing of permafrost, gasification of coal, as well as other uses. Some examples of subterranean heater arrangements include placing and operating electrical resistance heaters, microwave electrodes, gas-fired heaters or catalytic heaters in a bore hole of the formation to be heated. Other examples of subterranean heater arrangements include circulating hot gases or liquids through the formation to be heated, whereby the hot gases or liquids have been heated by a burner located on the surface of the earth. While these examples may be effective for heating the subterranean geological formation, they may be energy intensive to operate.
U.S. Pat. Nos. 6,684,948 and 7,182,132 propose subterranean heaters which use fuel cells as a more energy efficient source of heat. The fuel cells are disposed in a heater housing which is positioned within the bore hole of the formation to be heated. The fuel cells convert chemical energy from a fuel into heat and electricity through a chemical reaction with an oxidizing agent. U.S. Pat. Nos. 6,684,948 and 7,182,132 illustrate strings of fuel cells that may be several hundred feet in length. Operation of the fuel cells requires fuel and air to be supplied to each of the fuel cells and spent fuel (anode exhaust) and spent air (cathode exhaust) must be exhausted from each of the fuel cells. In order to do this, a fuel supply conduit and an air supply conduit are provided such that each extends the entire length of the string of fuel cells to supply fuel and air to each of the fuel cells. Similarly, an anode exhaust conduit and a cathode exhaust conduit are provided such that each extends the entire length of the string of fuel cells to expel anode exhaust and cathode exhaust from each of the fuel cells to the surface (e.g., the top of the bore hole of the formation).
One problem that is encountered is packaging of the anode exhaust conduit and the cathode exhaust conduit since space within the heater housing is largely needed for the fuel cells, fuel supply conduit, and air supply conduit. If the anode exhaust conduit and the cathode exhaust conduit are sized too small, there will be a very large pressure drop from the bottom of the heater to the surface of the formation, thereby resulting in significant pumping losses and inefficiency. In addition, high pressure in the anode exhaust conduit and the cathode exhaust conduit will result in high pressure within each fuel cell, particularly near the top of the heater. The ambient pressure within the heater housing will be much lower, thereby resulting in a pressure difference between the interior of the fuel cells and the interior of the heater housing. Accordingly, the fuel cells would need to be constructed in such a way as to withstand this pressure differential which leads to higher cost and manufacturing difficulty of the fuel cells. Alternatively, if the anode exhaust conduit and the cathode exhaust conduit are sized for minimal pressure drop, they would occupy a large portion of the heater housing, thereby reducing the size of the fuel cells and the amount of heat produced by the fuel cells.
A second problem that is encountered is that much of the heat generated by the fuel cells is captured within the anode exhaust and the cathode exhaust. Since the purpose of the heater is to heat the formation, it is desirable to utilize as much of the generated heat to heat the formation. While some of the heat of the anode exhaust and the cathode exhaust is transferred by radiation from the anode exhaust conduit and the cathode exhaust conduit, most of the heat of the anode exhaust and the cathode exhaust will return to the surface without heating the formation.
A third problem that is encountered is that in order for the fuel cells to generate the heat that is required to heat the formation, it may be necessary to operate the fuel cells at a high load condition where the internal losses are relatively high. Running the fuel cells at high load conditions may result in degradation of the fuel cells and a shortened operating life.
What is needed is a heater which minimizes or eliminates one of more of the shortcomings as set forth above.